Slush moulding is a process whereby hollow articles may be prepared from a TPE. The process typically involves heating a hollow mould into which is poured a TPE composition in the form of a powder and/or in the form of micropellets. The mould is then rotated and heated, whereby the TPE composition is sintered into a “skin”. When the mould is cooled, the finished product may be removed. The finished product may be of simple or complex form. A typical product is a dashboard cover or airbag cover used in automotive applications. Other automotive interior parts include glove door boxes, door panels, consoles etc. In this case, design flexibility, recyclability, both low and high temperature performance, light stability, weight savings, and high productivity are key performance indicators.
Currently, PVC is used for dashboard skins. PVC has reasonable melting/fusing properties (235 DEG C. and 80-150 seconds); suitably low viscosity allowing good flow; is available as powder; has good tensile strength; has good UV stability; has good scratch resistance; and is low in price. The disadvantage of PVC results from relatively high density (1.4), and its poor reputation with customers and legislators due to its impact on the environment, its use therein of specific plasticizers that cause fogging, odour and tackiness, and finally its lack of recyclability.
With the use of compositions based on hydrogenated styrenic block copolymers (HSBC), sometimes referred to as compounds, many of the PVC drawbacks can be overcome. This concept has therefore been the focus of innovative efforts. Although PVC is still the common material for commercially slush moulding. The current HSBC compositions are yet lacking in their ability to provide materials which easily and quickly flow and fuse in conventional commercial equipment yielding strong and flexible articles.
EP 673970 A concerns conveyor belts wherein a block copolymer composition is applied onto the fabric either as a hot melt or as a finely divided powder, which is subsequently fused into a continuous layer by heat treatment.
WO 99/32558 provides a thermofusible elastomer composition comprising: (a) 100 parts by weight of a selectively hydrogenated block copolymer component comprising an A′B′ block copolymer and a multiblock copolymer having at least two end blocks A and at least one mid block B wherein the A′ and A blocks are monoalkenyl arene polymer blocks and the B′ and B blocks are substantially completely hydrogenated conjugated diene polymer blocks, the number average molecular weight of the A′ and A blocks are in the range from 3,000 to 7,000 and the monoalkenyl arene content of the multiblock copolymer is in the range from 7 to 22 weight percent; (b) 20 to 50 phr of at least one high melt flow polyolefin; (c) 0 to 19 phr of a plasticizing oil selected from naphthenic oils and paraffinic oils; and (d) 0 to 45 phr of at least one poly(conjugated diene) block compatible resin wherein the elastomer composition has a particle size of 1400 micron or less. This composition is used in low shear processes.
WO 95/03447 concerns recyclable carpets, obtainable by means of the application of a powdered block copolymer composition, comprising at least: a) a block copolymer, containing at least two terminal poly(vinylaromatic) blocks and at least one internal poly(conjugated diene) block, the original ethylenical unsaturation in which may optionally be selectively hydrogenated, which block copolymer may optionally be grafted with a polar entity; b) a non-aromatic plasticizing oil; c) a terminal block compatible resin on the backside of a carpet fabric; and block copolymer compositions for the manufacture of such carpets.
EP 659831 A relates to a free flowing powder composition with a particle size of 800 μm or less comprising: (a) 100 parts by weight of a block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block and at least one hydrogenated or unhydrogenated poly(conjugated diene) block and having a monovinyl aromatic hydrocarbon content in the range of from 10 to 60 weight percent based on total weight of block copolymer; (b) 50 to 200 parts by weight of plasticizing oil; (c) 50 to 200 parts by weight of a poly(monovinyl aromatic hydrocarbon) block compatible resin; and (d) 0.1 to 10 weight percent based on total weight of composition of a dusting agent. Process for the preparation of the above free flowing powder composition, comprising first melt blending all components except the dusting agent and the blowing agent, if present, followed by cooling. The resulting composition is then pelletized or granulated and the granules or pellets thus obtained are cryogenically milled. Finally, the dusting agent and optionally the blowing agent are dry-blended with the powder resulting from the previous step. The free flowing powder composition is very useful for obtaining homogeneous layers having a smooth surface. These layers can be applied successfully for carpet backings, conveyor belts, bottle cap sealants.
JP 11092602 concerns a thermoplastic elastomer composition for powder slush moulding, made by compounding a polypropylene resin, a hydrogenated styrene butadiene rubber, a process oil, an elastomer (e.g. a styrene-(ethylene-propylene)-styrene block copolymer), an adhesion-improving agent (e.g. an acid modified polypropylene or a hydroxyl-containing polypropylene), an internal mold releasing agent (e.g. dimethylsiloxane) and optionally an organic peroxide.
Likewise, in JP 11342509 a thermoplastic elastomer composition is produced for powder moulding by adding at least a hydrogenated styrene/butadiene rubber (H-SBR), a process oil and an elastomer excellent in oil absorbing capacity to a polypropylene resin with MFR (melt flow rate JIS K720) of 100-800 g/10 min and kneading these components under heating.
In EP 811657 A a thermoplastic elastomer composition for powder slush moulding is described, made up of a polypropylene resin, hydrogenated styrene/butadiene rubber, a process oil and an elastomer excellent in oil absorbing power. A process for making that thermoplastic elastomer composition may include the addition of an organic peroxide to the above components and kneading the obtained mixture while heating.
U.S. Pat. No. 6,906,144 describes a polypropylene-based resin composition for powder slush moulding which is a mixture of a polypropylene homopolymer, copolymer, or terpolymer; an olefin rubber; a styrene rubber; a process oil; and a polypropylene wax, wherein the composition is excellent in low-temperature impact resistance and heat resistance and has no offensive odor. The composition can be formed by a cryofreeze pulverization method in the presence of liquid nitrogen into a powder having an average particle size of 200 to 300 μm suitable for powder slush moulding. The powder can be adhered to a polyurethane foamed layer and a surface treating agent without a primer coating. The powder is particularly suitable for forming the interior surface for manufacture of an automobile surface such as an instrument panel.
In JP 2003246910 a slush-moulding material is disclosed that is composed of a hydrogenated copolymer composition consisting of 50-97 weight percent hydrogenated copolymer (1) and 3-50 weight percent. thermoplastic resin (2). The hydrogenated copolymer (1) comprises a conjugate diene and a vinyl aromatic compound composition containing (a) >50 weight percent to ≤90 weight percent vinyl aromatic compound and (b) ≤40 weight percent vinyl aromatic polymer block in the copolymer having (c) 50,000-1,000,000 weight average molecular weight and (d) ≥75% hydrogenated double bonds of the conjugated diene compound in the copolymer. The hydrogenated copolymer composition is pulverized and used for powder-slush moulding to provide a moulded product suitable for the skin material
From EP 0733677 A a composition is known, that is suitable for use in rotational moulding, and that is based on a block copolymer of the type SEBS (having at least two polystyrene end blocks and a hydrogenated conjugated diene midblock). The thermoplastic material is in the form of a very fine powder or micropellets that in combination with a dusting agent flows and spreads to form a fused thermoplastic layer on the inner surface of a mould.
In EP 1396525 A similar compositions for soft sheet applications are described, comprising a blend of polypropylene and a styrenic elastomer. The composition may in the form of a powder and/or a micropellet form. The composition may comprise a ethylene copolymer. In the process of slush moulding, improved flow is achieved by the addition of a suitable flow additive.
In EP 1605002 A HSBC compositions are described having good processability, flexibility, weather resistance and various other properties. However a composition that excels in slush moulding is not yet disclosed,
In U.S. 2006100380 again a slush moldable TPO composition is provided, comprising a blend of PP, styrenic elastomer, a linear low density PE and an oil.
Likewise EP 2009053 A discloses a TPO composition having improved melt viscosity and methods of making the same.
Finally, in WO 03/066731 controlled distribution block copolymers are disclosed that may be compounded with a large variety of other polymers, including olefin polymers, styrene polymers, tackifying resins, and engineering thermoplastic resins. The compounds may, for instance, be used for slush moulding of automotive skins.
Despite the many patent references on compositions for slush moulding, still many commercial products are based on PVC. PVC has various disadvantages, which includes fogging, the release of volatile organic compounds, oil and plasticizers, odour, etc. These disadvantages could be overcome with a composition based on a SBC, but still the properties and/or the processability of these new SBC compounds are not good enough.
The properties that are required to for e.g. use in automotive applications (dashboards) typically include a tensile strength σ of 11±2 MPa and an elongation at break ε of ≥250%. In practice, a tensile strength σ≥10 is required. Other properties include hardness, density, abrasion resistance, scratch resistance, fogging, flammability and colour fastness to light. More specifically, in automotive applications (dashboards) the new composition should meet the following performing characteristics:
a hardness between 60 and 95 Shore A according to ASTM-D2240;
a tensile strength ≥8 MPa according to ASTM D412-C;
passing the compound melting test.
The compound melting test is carried out in order to determine the suitability of a composition to slush moulding/rotomoulding processing. The test indicates if a thermoplastic material fulfils certain prescribed melting behaviour characteristics when heated under controlled conditions. A specimen passes the test when both a visual and a thickness variation evaluation are indicated as positive. The compound melting test is described in more detail in the experimental section.
Moreover, in addition to the above properties also the appearance and feel is important. For instance, the final product should not have a plastic (sticky or oily) feel. It should not look “cheap” or of low quality, related to the consumers' impression when low quality, inexpensive plastics are used. Likewise, the final product should be free from pinholes, but also show no undesired flow patterns or similar irregularities.
In terms of processability, the requirement of great importance is the melt speed, which can be tested for instance by the method described hereinafter.
Herein we refer to melt viscosity at any given temperature as that property measured at low shear rates, such as that defined by zero shear rate viscosity. The melt viscosity of the thermofusible compositions for use in slush moulding should be, but is not limited to, in the range of 50 Pa·s to 250 Pa·s over the processing temperature range of 180 DEG C. to 260 DEG C. as measured at low shear rate such as that applied by parallel plate rheometer.
Interestingly, now a composition has been found that can be used without difficulty by the end users currently using PVC in slush moulding applications (no need of new equipment) whereby products are made at lower processing temperature and reduced cycle times, compared to PVC. The product is environmental friendly (halogen free, easy recyclable), has an excellent feel and an excellent combination of mechanical properties. The invention is thus based on a specific compound.